[MachOYAML] Allow handcrafting custom contents for DWARF sections.

This patch enables users to handcraft custom contents for DWARF
sections. If we specify the contents of DWARF sections both in the
'DWARF' entry and the 'content', yaml2obj will emit an error message.
In addition, this patch helps remove the restriction that only the
content of sections whose segname are __DWARF can be specified in the
"DWARF" entry.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D87126